Elm tree named Independence

ABSTRACT

An elm tree characterized by high resistance to Dutch Elm Disease.

The present invention relates to a new and distinct variety of anAmerican elm (Ulmus americana) named Independence. The varietyIndependence is characterized by a high degree of resistance to DutchElm Disease caused by Ceratocystis ulmi, and reduced susceptibility toStegophora ulmea (black leaf spot disease,) Verticillium dahliae(Verticillium wilt,) and Taphrina ulmi (leaf blister disease.)

The cultivar Independence is derived from a cross between the widelygrown cultivar "Moline" and pollen from a male tree initially grown atInterstate Nurseries of Hamburg, Iowa. It is one of a number of clonesknown as `American Liberty` elms demonstrating resistance to disease,particularly Dutch Elm Disease, and possessing a desirable uprightvase-shape and vigorous growth habit. Ramets of the female parent ofIndependence survive at Arlington, Wis. and other locations. The currentstatus of the female parent is not known. The male parent, designatedW185-21, was the result of disease resistance screening initiated atArlington, Wis. in 1960. The W185-21 tree died in 1973 after inoculationwith a particularly virulent mixture of C. ulmi from North Americansources.

General characteristics of the variety Independence include an uprightvase shape, dense foliation, and vigorous growth rate. It develops asubstantially upright main trunk during early growth stages with olderbranches becoming primarily horizontal at maturity. It propagates easilyand establishes well in field testing.

The accompanying drawing shows a spcimen tree of the new variety inearly fall coloration illustrating its general form and habit of growth.

General leaf size, shape, color, bark color/texture, and other growthcharacteristics are not distinctive and typical of North American whiteelms. The technical and descriptive information regarding leaves, bark,and other growth characteristics of the variety Independence tree areprovided herein in compliance with statutory disclosure requirements,and are not distinguishing features of the tree. The mature leavesexposed to full sun are a dark "parsley green" (00962) (Wilson, R. 1941.Horticultural Colour Chart. Vol. 2. British Colour Council, London.) ontheir upper sides, and are glabrous or slightly pubescent with scatteredtrichomes bent parallel to the surface and pointing toward the tip. Thelower surfaces of the leaves are "spinach green" (0960/3) (Wilson,supra) ranging from downy-pubescent to glabrous. Pubescence tends to bea juvenile characteristic found mainly on the most vigorous shoots ofthe tree, while leaves on less vigorous branches are always glabrous.All phases of pubescence can be found on a single tree.

Mature leaves are ovate-elliptical, unequal at the base, sharplyacuminate at the tip, and doubly serrate at the margins. Many of thesprout leaves develop a vestigial petiolar leaflet. Often these leafletsare fairly large, resulting in near compound-leaf form.

Fall leaf color characteristics are variable. However, the leavesfrequently turn a bright golden yellow with abscission occurring in lateSeptember or early October (in south central Wisconsin.)

Specific leaf characteristics as derived from greenhouse tests aredescribed as follows in Table 1.

                  TABLE 1                                                         ______________________________________                                        Average leaf length = 15.0 to 16.5 cm                                         Average leaf width = 8.5 to 11.5 cm                                           Average petiole length = 0.8 to 0.9 cm                                        Average growth rate = 0.85 cm/day*                                            ______________________________________                                         *(Growth rate was measued during log phase on greenhouse grown plants in      five inch pots from dormancy broken oneyear-old rooted cuttings bud prune     at bud break to allow the growth of a single dominant shoot. The              comparison was carried out at tempera tures of 24° C. day              18° C. night under a 16 hour day supplied by supplemental              illumination.)                                                           

Newly formed bark develops in varying shades of brown, but after aging,changes to a gray color. With maturity, cracks having broad, flat, andscaly ridges are formed in the bark.

ASEXUAL REPRODUCTION

Propagation can be carried out in a greenhouse using root cuttingsallowed to sprout in moist sphagnum peat and sand. Propagationoriginally took place at the University of Wisconsin in Madison, Wis.under the direction of the Department of Plant Pathology. Greenhousetemperatures are adjusted to 180° C. at night, and 24° C. during theday, with 16 hour days supplied by supplemental illumination. Greenwoodcuttings dipped in rooting hormone can be transplanted as rootedcuttings after approximately 20 days in a perlite-peat rooting mediumunder fine intermittent mist.

DETERMINATION OF RESISTANCE TO DUTCH ELM DISEASE

Methods for screening elm introductions and breeding progeny for DutchElm Disease resistance have been previously described (Lester, D. T.,and E. B. Smalley, 1972. Improvement of Elms Through InterspecificHybridization With Asian Species. IUFRO Genetics-Sabrao Joint Symposia,Tokyo 1972. C-5 (V):1-10; Smalley, E. B., and A. G. Kais, 1966. SeasonalVariations in the Resistance of Various Elm Species to Dutch ElmDisease, in H. D. Gerbold et al., (eds.) Breeding Pest Resistance Trees,Pergamon Press, NY, pp. 279-292.)

In the case of the variety Independence and other so-called 500 seriesAmerican Liberty elm trees, control-pollinated seedlings were grown forone year in outdoor seedbeds and transplantd to field locations in thesecond year. Thereafter, the seedlings were inoculated with C. ulmi thefollowing spring. The inoculum consisted of a mixture of isolates fromWisconsin, Kansas, Massachusetts, Illinois, Nebraska, Minnesota,Connecticut, and Maine. Selected survivors which developed less than 10%crown damage were then increased vegetatively in 1975 and grown inreplicated field plantings for another two years. In 1978, selectedramets of each 500 series elm were inoculated with the C. ulmi mixtureat differing periods during the growing season. Observations on diseasedevelopment were recorded periodically for the following year. Theresults of these tests on the Independence, therein identified as cloneW-510, are set forth in Table 2, together with results of tests on otherclones. At the top of the table, the dates of inoculation are listed. At"T", the number of ramets without symptoms after inoculation versus thetotal number of ramets inoculated with C. ulmi is shown. Beneath thesenumbers is a calculation of the percentage of ramets that did not showsymptoms of Dutch Elm Disease after inoculation.

                  TABLE 2                                                         ______________________________________                                                 Ramets without symptoms.sup.y                                                 Dates of inoculation.sup.x                                           Clone      5/25    6/1     6/7   6/14  6/21                                   ______________________________________                                        Unselected.sup.u                                                              W-411-3 T      3/7     0/8   0/6   0/4   1/12                                         %      42.9    0     0     0     8.3                                  W-412-1 T      1/12    1/9   2/11  0/5   0/12                                         %      8.3     11.1  18.2  0     0                                    `American Liberty`- 500 series.sup.w                                          W-502   T      --      0/2   --    --    --                                           %      --      0     --    --    --                                   W-503   T      0/6     0/6   0/4   1/5   2/6                                          %      0       0     0     20    33.3                                 W-505   T      --      --    --    --    --                                           %      --      --    --    --    --                                   W-507   T      5/6     --    --    0/1   1/2                                          %      83.3    --    --    0     50                                   W-510   T      2/11    3/4   0/12  2/8   2/5                                          %      18.2    75    0     25    40                                   `American Liberty`- "Topeka" Cultivar.sup.v                                   M-8     T      1/5     1/8   3/5   2/7   6/8                                          %      20      12.5  60    28.6  75                                   ______________________________________                                                 Ramets without symptons.sup.y                                                 Dates of inoculation.sup.x                                                                                  All                                    Clone      6/28    7/6     7/18  8/3   Dates.sup.z                            ______________________________________                                        Unselected.sup.u                                                              W-411-3 T      --      0/6   0/9   3/3   7/55                                         %      --      0     0     100   12.73                                W-412-1 T      --      0/10  1/9   2/2   7/70                                         %      --      0     11.1  100   10.00                                `American Liberty`- 500 series.sup.w                                          W-502   T      --      4/6   4/6   --    8/14                                         %      --      66.7  66.7  --    57.14                                W-503   T      0/2     1/5   3/3   2/2   9/39                                         %      0       20    100   100   23.1                                 W-505   T      --      5/5   1/1   --    --                                           %      --      100   100   --    --                                   W-507   T      2/2     3/5   2/2   --    13/18                                        %      100     60    100   --    72.2                                 W-510   T      6/9     4/9   7/8   1/1   27/67                                        %      66.7    44.4  87.5  100   43.3                                 `American Liberty`- "Topeka" Cultivar.sup.v                                   M-8     T      8/8     5/5   7/8   2/2   35/56                                        %      100     100   87.5  100   62.5                                 ______________________________________                                         .sup.x Mixed inoculum from isolates of Ceratocystis ulmi from Wisconsin,      Kansas, Massachusetts, Illinois, Nebraska, Minnesota, Connecticut and         Maine.                                                                        .sup.y Trees planted as rooted cuttings, 3 plants per clone per block, 15     clones per block, and 36 to 40 blocks per series. (Many clones are            incompletely represented in blocks because of transplant mortality or         shortage of plants.) Data is derived fr om 1971-72, 1974-75 and 1978-79       series and not all clones in the studies are shown.                           .sup.z Underlined data indicates the approximate period of highest            susceptibility.                                                               .sup.u Data from 1971-72                                                      .sup.v Data from 1973-74                                                      .sup.w Data from 1978-79                                                 

As indicated in Table 2, the Independence elm presents high resistanceto Dutch Elm Disease as compared with other varieties. It is not asresistant as some varieties, but its susceptible period (early June) isshorter than that of other tested varieties. The physiological basis forresistance to C. ulmi is not presently understood. Preliminary researchsuggests an ability of the tree to illicit antifungal phytoalexins inresponse to the presence of C. ulmi spores. Research as described inSmalley, E. B., N. S. Ehlke, E. E. Clark, and S. H. Mai. 1982. Condialreactions in Ulmus following inoculation with Ceratocystis ulmi.Phytopathology 72:981 suggests that such capability increases with plantage, both seasonally and with developing maturity.

Field observations also suggest that the Independence elm is not undulysusceptible to black leaf spot disease caused by Stegophora ulmea (Schw:Sydow and Sydow). It also exhibits tolerance to Verticillium dahliaeKleb (the casual agent of Verticillium wilt) and Taphrina ulmi (leafblister diseases.) Furthermore, because the growth characteristics ofthe tree involve an upright growth habit and acute branching angle, ithas demonstrated resistance to severe winds and ice storms.

I claim:
 1. A new and distinct variety of elm tree as shown anddescribed herein primarily characterized by a short period ofsusceptibility to Dutch Elm Disease, vigorous growth rate, and ease ofpropagation.